Rose black spot

Symptoms

Black to dark-brown spots with ragged margins on rose leaves, surrounded by yellowing of the leaf blade. Infected leaves drop in large numbers, weakening the plant and reducing flowering. The disease intensifies in wet seasons and where air circulation is poor. The spots are usually round, from a few millimetres to about 1-1.5 cm across, and their most important diagnostic feature is the radially fringed, feathery margin - the mycelium spreads beneath the leaf cuticle in all directions, producing the characteristic 'star-shaped' edge (hence the German name Sternrußtau). Under heavy infection the spots merge into larger, irregular patches, and the surrounding yellowing spreads across more and more of the blade until the whole leaf turns yellow and drops. Small, dark conidial fruiting bodies (acervuli) appear on the upper surface of the spots, while the underside of the leaf remains free of any coating. Symptoms appear first on the lower, oldest leaves and gradually move up the shrub, because the spores are carried by water splashing up from the soil and from lower leaves. Small, purplish-red discolorations and blotches may appear on young, green shoots. Prolonged wetting of the leaves for several hours is required for the spores to germinate and infect, which is why the disease breaks out after warm, rainy spells. Repeated defoliation exhausts the shrub, limits flowering and worsens winter hardiness.

Treatment

Remove and destroy infected and fallen leaves (the source of infection for the following year), and apply fungicides in cases of heavy infection. Thinning cuts improve air movement through the shrub. Fallen leaves are worth collecting throughout the season, not only in autumn - every infected blade is a source of spores for further infections; diseased leaves should not be composted in a garden heap (the temperature is too low to kill the fungus) but instead removed from the garden or buried deeply. During pruning, remove shoots bearing spots and dying shoots. Under heavy disease pressure, fungicides are used in a preventive-curative programme: contact products (sulphur- or copper-based) form a protective barrier on the leaf surface and work best before infection, while systemic products (triazoles) and quasi-systemic ones (strobilurins) penetrate the tissue and control the mycelium already developing inside the leaf. It is essential to rotate products with different modes of action (different FRAC groups), because Diplocarpon rosae readily develops resistance to a single group. Treatments begin in early spring, when the weather favours infection, and are repeated every 7 to 14 days during wet periods. At the same time, leaf wetting is reduced (watering at the base) and balanced fertilisation is maintained, because lush shoots over-fed with nitrogen are more susceptible.

Prevention

Choose resistant varieties, plant with airy spacing, water at the base in the morning, rake up leaves in autumn, and use potassium-rich feeding to strengthen tissues. When choosing cultivars it is worth reaching for roses of proven resistance (for example those awarded the ADR mark) - this is the cheapest and most durable form of protection. The site should be sunny and airy, and the spacing loose enough for the leaves to dry quickly after rain and dew; overly dense plantings and proximity to walls that restrict air movement favour the disease. Water at the base of the shrub, preferably in the morning, avoiding wetting the leaves, which prolongs the period they stay wet. Hygiene is very important: raking up leaves in autumn and early spring and mulching the soil around the shrubs, because a layer of mulch limits the splashing of spores from the soil surface onto the lower leaves. Balanced fertilisation with an adequate proportion of potassium strengthens the tissues and raises resistance, whereas an excess of nitrogen lowers it. Thinning cuts improve air circulation within the crown, and preventive spring sprays (before symptoms appear) reduce disease pressure during the season.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.